Leader of the far-right National Front (FN), Marine Le Pen, says French voters rejected the “old political class” in regional elections that put her party top.
Nearly one-third of voters backed the anti-immigration FN, which won in six out of France’s 13 regions.
Despite the FN surge, a second round on 13 December will be the decider.
It was the first electoral test since last month’s Paris attacks, in which jihadist gunmen killed 130 people.
The nationalist FN got about 28%, ahead of the centre-right Republicans party led by former President Nicolas Sarkozy, which polled just under 27%, and the governing Socialist Party (PS), trailing with 23.5%.
The PS said it was withdrawing from the second round in two regions, in the north and south, to try to block a run-off victory for the FN.